Bug 907562 - [Messages] the "unread" bit is not always removed when we read a thread - r=julienw

https://bugzilla.mozilla.org/show_bug.cgi?id=907562

* The path for switching from #new to #thread=xxx didn't call ThreadListUI.mark
* Simplified the path difference, only removing the "slide" effect from going from #new to #thread=xxx